Discontinuous debacterization methods are known that comprise a treatment enclosure fitted internally with heating contact walls and with means for injecting steam under pressure, such methods making use of installations of large dimensions with high production costs.
Continuous debacterization methods are also known that comprise a treatment enclosure having a double-walled conveyor screw located therein, with a heat-conveying fluid being caused to pass therethrough and with steam also being injected into the enclosure. Because the steam condenses, it is found that the substance becomes wet, thus requiring it to be subjected subsequently to drying treatment.
The same drawback is to be found with fluidized vibrating bed type conveyors or with pressurized gravity tube systems having the substances flowing therethrough. A high operating pressure, e.g. of the order of 10 bars, is sometimes even used, as described in document FR-A-2 788 336, thereby constituting a considerable constraint on implementing the method.
Electric current conveying vibrating tube type conveyors have also been used, as described in document WO-A-99/65332, however the heat exchange, which is mainly conductive, can lead to very large temperature gradients, thus giving rise to sticking as a result of the substance burning against the inside wall of the tube when the setpoint temperature lies above a degradation temperature for the substance.
Proposals have also been made for conveyor systems using a screw that conveys electrical current directly, as described in document WO-A-99/39549 in the name of the Applicant. In such systems, the substances are conveyed while simultaneously being stirred and heated by coming directly into contact with a conveyor screw that is itself heated by the Joule effect.
More recently, the above-mentioned system has been further improved by providing for wet steam at atmospheric pressure to be injected into the closed conveyor enclosure, which steam is previously superheated prior to penetrating into the enclosure by a member for heating gaseous fluid and that is located outside the enclosure. It is then possible to control the system in such a manner that the temperature of the superheated wet steam inside the closed enclosure at atmospheric pressure remains substantially equal to the temperature of the heating contact walls that act directly on the substances while they are being conveyed. For greater detail, reference may be made to document WO-A-2009/133321 in the name of the Applicant.
Those techniques give results that are entirely satisfactory with substances in divided solid form that present a mass with good fluidity. This applies for example when treating peppercorns or coriander seeds or hazel nuts. It then suffices to organize effective stirring of the substances while they are being conveyed in order to obtain good transfer of heat, with balanced temperature conditions being achieved very quickly, which is favorable to the treatment insofar as the risk of denaturing the substances is reduced.
Nevertheless, when the mass of substances in divided solid form presents poor fluidity, thus encouraging the formation of slopes, as happens with numerous food or medicinal powders, a technical difficulty is encountered that stirring on its own is not capable of solving. Either the substances cannot be treated because of temperature levels that are too high thereby leading to the substances being denatured, or else the debacterization thereof is insufficient to ensure quasi-total destruction of molds and spores.